Introduction
The recent tragic deaths of ten workers resulting from a mine accident in Pepper Creek, Konawaruk has brought to light the intrinsic issues with mining at depths below grade (exceeding 15m). This accident accounted for more lives than the world famous Omai cyanide spill, which was labelled as Guyana’s worst mine disaster.
Accidents in alluvial mining operations in Guyana are primarily caused by unstable slopes. Detailed geotechnical data is often a major unknown factor in open pit mines, the lack of which constitutes a significant risk that can lead to catastrophic failure.
For example, pit excavation may progress vertically without sufficient lateral opening, as was done at the Pepper Creek mine in Konawaruk. In addition, the mine face is not usually benched with increasing depth and undercutting is a common practice. This, coupled with the effects of rainfall, vibrations from the engines and vehicles and the lateral stress caused by superimposed loading, creates a recipe for slope failure.
Information on guidelines including the specific requirement for the hiring of competent technical personnel to undertake and supervise these activities are presented to miners on a quarterly basis via seminars/presentations in each mining district by the Commission’s field staff. In this article, the Commission will outline the concept of deep alluvial mining and the various factors to be considered when such mining methods are employed.
Background
The Guyana Geology & Mines Commission (GGMC) is the regulatory body created in 1979 for administering the Mining Act (1989), the Mining Regulations (1922) and the Mining (Amendment) Regulations (2005). The role of the Commission includes the establishment of safe mining environment for small, medium and large scale mines.
In Guyana, the number of operating opencast mines is steadily increasing. This is due to a low gestation period, higher productivity and high rates of return on investment. However, opencast mining attracts environmental concerns such as solid waste management, land degradation, health and safety and socio-economic problems. In addition to that a large number of opencast mines, whether medium or small, are now reaching to deeper mining depths. As a result, analysis of stability of operating slopes and ultimate pit slope design are becoming a major concern. Slope failures cause loss of production, extra stripping cost for recovery and handling of failed material, dewatering the pits and sometimes serious injuries or death to mine workers.
The number of medium-scale mines are increasing yearly given the access and affordability to heavy earth-moving equipment such as backhoes and bulldozers. These equipment allow operators to mine at increasing depths, given the rapid depletion of the shallow alluvial deposits that were worked over the last two decades. These practices have brought new concerns of mine safety within the industry. (A medium-scale mine is defined as one where more than 200m3 but less than 1000m3 of material is processed as an aggregate, inclusive of any overburden, in a continuous 24-hour period, according to the Mining (Amendment) Regulations, 2005.)
Medium-scale mines progressing beyond 15m are of concern to the Commission from a safety point of view. The Commission has published guidelines on such mines, including the necessary steps to be taken when such depths are encountered.
Accidents in alluvial mining operations in Guyana, are primarily caused by unstable slopes.
The US Occupational Safety and Health Organization (OSHA) has recommendations for maximum permissible slope for different classes of materials in subpart P of part 1926 in its guidelines. Excavations equal to or exceeding 20 feet require benching to ensure slope stability and prevent mine accidents. For the four identified classes of materials: stable rock, Type A soils, Type B soils and Type C soils (OSHA Technical Manual), respective maximum allowable slope angles were determined at 90o, 53o, 45o and 34o. However, it was also identified that the actual slope to which the face is excavated shall not be steeper than the maximum allowable slope. To make such technical assessments, the mine operator is required to secure the services of a duly qualified individual to ensure mine safety. In addition, frequent visual inspections are required to identify tension cracks which usually form at a horizontal distance of 0.5 to 0.75 times the depth of the excavation or bench.
Most mine owners and operators may be efficient in the administration aspect of mining but have little knowledge of the technical requirements or how to achieve such. Hence, it is vital that professional services be sought to ensure safety of medium-scale gold and diamond alluvial mines in Guyana.
Solution to the problem
Slope failure in alluvial mines in Guyana can be prevented through technical assessment prior to and during mining. The basic tests required to be conducted on an alluvial deposit prior to mining and during the life of the mine are, but not limited to, visual assessments, particle size analysis, Atterberg limits, unconfined/uniaxial compressive strength or shear vane testing (in-situ test) (OSHA Technical Manual).
Medium-scale alluvial gold and diamond mines in Guyana therefore require competent, trained persons (preferably professional engineers) to provide technical services in order to ensure safe mining is practiced during the life of the mine. At such scale of mining, the geotechnical properties of the material become of critical importance. The results of these analyses coupled with the impact of the external factors can be analysed by an engineer to determine bench parameters, sloping and ultimately, the pit limits.
Of particular importance are the particle size distribution, moisture content, permeability, cohesion, shear strength and angle of repose. (The shear strength is the strength of the material against the load of superimposed material or equipment that may cause the material to produce a sliding failure. The angle of repose is the angle of the steepest slope at which the material will remain stable.) External factors such as loading, rainfall and vibrations are also important as they may affect the geotechnical properties. A technician may provide services for soil sample collection and testing; however, an engineer is required to analyse the results and determine the safety parameters of the pit.
The Commission’s monthly monitoring through its Environmental Monitoring Campaign conducted on all mining operations across the six mining districts has adequately addressed the issue of mine safety through its education and awareness programmes/seminars. However, it must be emphasised that such programmes are not designed to teach or transfer knowledge to the miners how to interpret and analyse geotechnical data, but rather to highlight the importance of maintaining a safe working environment in the mines. These interpretation and analysis are to be undertaken by trained professionals.
Factors to be considered in assessment of stability are done through a ground investigation exercise, where essential borehole information must be obtained. This information will give details of the strata, moisture content and the standing water level and shear planes. Piezometer tubes are installed into the ground to measure changes in water level over a period of time. Ground investigations also include:-
- In-situ and laboratory tests;
- Aerial photographs;
- Study of geological maps and reports to indicate probable soil conditions; and,
- Visiting and observing the slope.
The general requirements for such mines start with the concept of mine planning. There are two basic objectives in mine planning: first to determine the ore reserve and second the extent of the ore body, where the ultimate geometry of the pit is defined.
Maintaining pit slope angles that are as steep as possible is of vital importance to the reduction of stripping (overburden), which will in turn have direct consequences on the economics of the mining operation. Design of the final pit limit is thus, governed not only by the ore grade distribution and the production costs, but also by the overall material strength and stability. The potential for failure must be assessed for a given mine layout and incorporated into the design of the ultimate pit. Against this backdrop, there is a strong need for good practices in slope design and management so that suitable corrective actions can be taken in a timely manner to minimize the slope failures.
In concluding, opencast mining is seen as a cost effective mining method employed by medium and small scale gold and diamond miners in Guyana to process high volumes of material; it employs a fair degree of mechanization. Mining depths in these open pits have increased steadily during the last decade, where the inherent risk of slope instability has been highlighted. It is therefore imperative that mine operators retain the service of trained professionals to conduct geotechnical evaluation of the mine and to provide management services, so as to remain in the realm of safety, when employing these mining methods.
The Commission continues to make its Mines Technical Section available to the public. This is in addition to staff in the various mining districts who are providing essential support to the mining industry on key issues such as legislation, safety, good health practices and pollution abatement measures.